212 METHODS IN TEACHING 



ten a toy balloon to a glass tube ten inches in length, and 

 thrust the other end upward through a bored cork that will 

 fit a large florence flask. Thrust the end having the balloon 

 attached into the flask until the cork fits snugly the mouth 

 of the flask. Try to inflate the balloon by forcing air into 

 the outer end of the tube. It can not be done. Why not? 

 Withdraw the cork a very little way, inflate the balloon, and 

 replace the cork. The balloon remains inflated. Why? 

 Take two test-tubes, one of which is enough smaller than 

 the other to slide easily into it. Fill the larger one nearly 

 full of water ; put the end of the smaller tube into the larger 

 one until it displaces the water to a depth of about an inch 

 and a half. Invert the tubes in this position over a vessel 

 to catch the water. As the water slowly trickles out of the 

 larger tube, air pressure forces the smaller one into it. 

 This is a very striking experiment for pupils of any grade 

 above the fourth. 



Another that holds interest quite as well is the following : 

 Secure a board, two feet long, ten inches wide, and half 

 an inch thick. It should be of tough wood. Place it upon 

 a table or desk so that nearly half of it projects over the 

 edge. Now spread two or three thicknesses of newspaper 

 over the end that is on the table. Direct one of the stronger 

 boys of the class to strike a quick, heavy blow downward o 

 the projecting end. The class, as well as the boy, will 

 greatly surprised to find that the board is scarcely moved; 

 owing to air pressure acting upon the board and paperj 

 The paper, of course, simply prevents air from passinj 

 quickly under the board. Thus the blow meets the resis! 

 tance of the pressure on the upper surface of the board, th; 

 whole of which in this case would amount to nearly a tori 



